The present invention is a catalyst of a metal heteropoly acid salt that is insoluble in a polar solvent on a non-metallic porous support and method of making.
As used herein, xe2x80x98dispersedxe2x80x99 or xe2x80x98dispersionxe2x80x99 refers to the degree in which the catalyst is uniformly coated on a support surface as defined in Fundamentals of Industrial Catalytic Processes, R. J. Farrauto and C. H. Bartholomew, Blackie Academic and Professional Press (New York, N.Y.) 1997, p. 733. Specifically, dispersion (D) is defined there as xe2x80x9cthe fraction of atoms of a phase exposed to the surface. D=NS/NT, where NS is the number of surface atoms and NT the total number of atoms of a given kind. Dispersion increases with decreasing crystallite diameter approaching unity at a diameter of 1 nm.xe2x80x9d Thus, a catalyst material is said to be dispersed or highly dispersed if substantially all of the catalyst material uniformly coats the surface of a non-metallic support material. Conversely, low dispersion or non-dispersed indicates that a majority fraction of the catalyst material is present in the form of large clusters and/or as a distinctly separate phase from the non-metallic support material.
Homogeneous acid catalysts such as sulfuric acid and aluminum chloride are currently used to catalyze many industrially important reactions. Although these homogeneous acid catalysts are catalytically efficient, they are not environmentally benign and create disposal problems.
Demands for a cleaner environment are motivating the chemical and petrochemical industries to develop alternative catalyst systems and/or processes to meet more stringent regulations. One particular area that has attracted considerable attention recently involves the replacement of HF and H2SO4 liquid acids in commercial alkylation units by more environmentally benign heterogeneous solid acids. Although current homogeneous catalysts are efficient, their corrosive and toxic nature provides potential environmental hazards and presents operational problems, including difficulty in separation, recovery and reutilization, that results in higher capital costs. Among many solid acid systems, heteropoly acids (HPA) (also known as polyoxometalates, POMs for short) [C. L. Hill, guest editor, Chemical Reviews 98 (1998) 1-387) with Keggin anion structures have received considerable attention due to their simple preparation and strong acidity. Specifically, 12-tungstophosphoric acid (H3PW12O40), denoted as TPA hereafter, is among the most extensively studied since it possesses the highest Brxc3x6nsted acidity, stronger than that of 100% sulfuric acid, which results from minimized charge on the anion surface. However, to date, low efficiency due to low surface area, rapid deactivation and relatively poor thermal stability are some of the major problems associated with these TPAs in conventional bulk acid forms.
Attempts to improve the efficiency of these materials have been made by supporting tungstophosphoric acid (TPA) on various high surface area supports (G. I. Kapustin, T. R. Brueva, A. L. Klyachko, M. N. Timofeeva, S. M. Kulikov and I. V. Kozhevnikov, Kinet. Katal., 31 (1990), 1017: L. R. Pizzio, C. V. Caceres and M. N. Blanco, Appl. Catal. A: General, 167 (1998) 283) and, more recently, on mesoporous silica with ordered pore structures (I. V. Kozhevnikov, A. Sinnema, R. J. J. Jansen, K. Pamin and H. van Bekkum, Catal. Lett., 30 (1995) 241: I. V. Kozhevnikov, K. R. Kloetstra, A. Sinnema, H. W. Zandbergen and H. van Bekkum, J. Mol. Catal. A: Chemical, 114 (1996) 287: T. Blasco, A. Corma, A. Martinez and P. Martinez-Escolano, J. Catal., 177 (1998) 306: C. T. Kresge, D. S. Marler, G. S. Rav and R. H. Rose, U.S. Pat. No. 5,366,945, 1994). Kapustin, et al. reported that acidity of the supported TPA decreased in the following order: SiO2 greater than xcex1-Al2O3 greater than carbon. They concluded that the strong interaction between TPA and carbon might have resulted in the decomposition of the Keggin structure. Thus, silica is a suitable support material: likely due to its intrinsic inertness (Y. Izumi, R. Hasebe and K. Urabe, J. Catal., 84 (1983) 402; J. B. Moffat and S. Kasztelan, J. Catal., 109 (1988) 206; C. Rocchiccioli-Deltcheff, M. Amirouche, G. Herve, M. Fournier, M. Che and J. M. Tatibouet, J. Catal., 126 (1990) 591.).
Recently, mesoporous silica known as MCM-41, first developed by researchers at Mobil (J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz and C. T. Kresge, J. Am. Chem. Soc., 114 (1992) 10834; C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710.), has been used to support TPA clusters to take advantage of its uniform pore size and highly ordered structures. More recently, we have reported that acid neutralization of the mesoporous silica support assisted in preserving the Keggin structure even at TPA loadings as low as 10 wt % (Y. Wang, A. Y. Kim, X. S. Li, L. Wang, C. H. F. Peden and B. C. Bunker, ACS Book on Shape-Selective Catalysis, in press (1999).). Although, we have observed an enhancement in resistance to leaching of TPA by water when mesoporous silica was used as the support instead of amorphous silica, this was likely due to steric constraints rather than a direct improvement in the grafting of the TPA clusters on the surface.
Another approach to improve efficiency of HPA catalysts is by exchanging the acidic protons with large cations such as Cs. One particular Cs heteropoly acid salt, Cs tungstophosphoric acid (Cs2.5H0.5PW12O40), shows extremely high aromatic alkylation activity with non-polar reactants due to much enhanced surface area at this stoichiometry. Unfortunately, all bulk Cs heteropoly acid salts are micron-sized particles and can not be practically used in a fixed-bed type or a slurry type reactor because of pressure drop or filtration problems. Furthermore, the insolubility of the Cs heteropoly acid salts makes conventional catalyst preparation via aqueous impregnation impossible.
Soled et al (S. Soled, S. Miseo, G. McVicker, W. E. Gates, A. Gutierrez, and J. Paes, Catalysis Today, 36 (1997) 441-450) attempted to disperse Cs tungstophosphoricacid salts on silica supports in an effort to provide an environmentally friendly, kinetically efficient catalyst. Their method involves two-step impregnation via incipient wetness method i.e., impregnating catalyst supports with an aqueous Cs2CO3 solution followed by the impregnation/precipitation of an aqueous heteropoly acid solution. Their activity and sample characterization results showed that Cs heteropoly acid salts were non-dispersed with no measurable dispersion due to the high mobility of Cs cations during the second-step impregnation with the aqueous heteropoly acid solution. Consequently, the supported Cs heteropoly acid salts were not efficient in catalyzing aromatic alkylation reactions.
Thus, there remains a need for a catalyst that is environmentally friendly and catalytically efficient. More specifically, there is a need to develop a practical way for preparing engineered metal heteropoly acid salt catalysts.
The present invention includes a catalyst having
(a) a non-metallic support having a plurality of pores;
(b) a metal heteropoly acid salt that is insoluble in a polar solvent on the non-metallic support; wherein at least a portion of the metal heteropoly acid salt is dispersed within said plurality of pores.
The present invention also includes a method of depositing a metal heteropoly acid salt that is insoluble in a polar solvent onto a non-metallic support having a plurality of pores. The method has the steps of:
(a) obtaining a first solution containing a first precursor of a metal salt cation;
(b) obtaining a second solution containing a second precursor of a heteropoly acid anion in a solvent having a limited dissolution potential for said first precursor;
(c) impregnating the non-metallic support with the first precursor forming a first precursor deposit within the plurality of pores, forming a first precursor impregnated support;
(d) heating said first precursor impregnated support forming a bonded first precursor impregnated support;
(e) impregnating the second precursor that reacts with the precursor deposit and forms the metal heteropoly acid salt.
An advantage of the present invention is that metal heteropoly acid salts are grafted on non-metallic porous supports. It is a further advantage that both the activity data and characterization results confirm that the metal heteropoly acid salts are highly dispersed on non-metallic supports. An additional advantage specific for Cs heteropoly acid salts is that the unique characteristics of bulk Cs tungstophosphoric acid (Cs2.5H0.5PW12O40), including high specific surface area and enhanced thermal stability, can be achieved at lower Cs stoichiometry, thus sacrificing fewer acidic protons. In addition, the porosity of non-metallic supports (for example, mesoporous silica) can be tailored to add shape selectivity to the catalytic reactions of interest.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.